This invention relates generally to a development apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to a two component development system wherein a donor roll is loaded with triboelectrically charged toner particles, and subsequently the toner is charged by a corona device.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam, an LED array or an original document being reproduced. By selectively discharging certain areas on the photoconductor, an electrostatic latent image is recorded on the photoconductive surface. This latent image is subsequently developed by charged toner particles supplied by the development sub-system.
Powder development systems normally fall into two classes: two component, in which the developer material is comprised of magnetic carrier granules having toner particles adhering triboelectrically thereto and single component, which typically uses toner only. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The operating latitude of a powder xerographic development system is determined to a great degree by the ease with which toner particles are supplied to an electrostatic image. Placing charge on the particles, to enable movement and imagewise development via electric fields, is most often accomplished with triboelectricity. However, all development systems which use triboelectricity to charge toner, whether they be two component (toner and carrier) or mono-component (toner only), have one feature in common: charges are distributed non-uniformly on the surface of the toner. This results in high electrostatic adhesion due to locally high surface charge densities on the particles. Toner adhesion, especially in the development step, is a key factor which limits performance by hindering toner release. As the toner particle size is reduced to enable higher image quality, the charge Q on a triboelectrically charged particle, and thus the removal force (F=QE) acting on the particle due to the development electric field E, will drop roughly in proportion to the particle surface area. On the other hand, the electrostatic adhesion forces for tribo-charged toner, which are dominated by charged regions on the particle at or near its points of contact with a surface, do not decrease as rapidly with decreasing size. This so-called "charge patch" effect makes smaller, tribo-charged particles much more difficult to develop and control.
Jumping development systems, in which toner is required to jump a gap to develop the electrostatic latent image, are capable of image quality which can be superior to in-contact systems, such as magnetic brush development. Unfortunately, they are also much more sensitive to toner adhesion. In fact, high toner adhesion has been identified as a major limitation in jumping development. Up to now, mechanical and/or electrical agitation of toner have been used to break these adhesion forces and allow toner to be released into a cloud for jumping development. This approach has had limited success, however. More agitation often releases more toner, but high adhesion due to triboelectric charging still dominates in toner cloud generation and causes unstable development. For full color printing system architectures in which the complete image is formed on the image bearing member, an increase in toner delivery rate produces a highly interactive toner cloud, which disturbs previously developed particles on the latent image. This erases many of the original benefits of jumping development for color xerographic printing for the so-called image-on-image (IOI) architecture. Again, as the toner size is reduced, the above limitations become even more acute due to increased toner adhesion.
Given that charged particle adhesion is a major limiting factor in development with dry powder, it has been a goal to identify toner charging and delivery schemes which keep toner adhesion low. Clearly, the adhesion of the charged toner depends sensitively on the method used to charge the particles.
Another problem is that it is extremely difficult to attain stable charge levels of 40 .mu.C/g that are required for uniform loading and reloading of the donor roll with conductive development materials. Stable charge levels of that magnitude can be attained with insulative carrier but these lead to severe reload defects. The developer cannot load sufficient toner onto the donor roll to compensate for the toner developed on the previous revolution. Conductive carrier designs have been identified that have sufficiently high initial charge levels, but these have all been very unstable with time.
For optimum development of edges in cloud based systems, a charge level of 40 .mu.C/g or higher is desirable. However, it is extremely difficult to attain stable charge levels of that magnitude with conductive development materials, which are required for uniform loading and reloading of the donor roll.
As a result, non-interactive development for Image-on-Image (IOI) full color printing systems suffers from serious limitations on development latitude. The primary constraint is that development is strongly dependent on the adhesion of the toner. To make matters worse, toner adhesion often fluctuates significantly with the changing operating conditions of the hardware and the state of the developer materials, causing both long and short time stability problems.
In Hybrid Scavengeless Development (HSD) systems used for non-interactive development in IOI color printers, a series of AC biased wires are closely spaced from a donor roll to detach toner and form a cloud. The HSD system has limitations due to mechanical vibration of the wires (strobing) and wire contamination due to the trapping and attachment of debris (e.g. fibers or toner particles). Both problems result in image noise and visible print defects. Wire motion issues also limit the maximum process width, because longer wires exacerbate the strobing problem.
An object of the present invention is to remove problems associated with toner adhesion and wires employed in such scavengeless development.